Magmatic epidote in Quanganglintou gneissic dacodiorite from the Changle-Nan’ao structural zone: Discovery and geological significance

doi:10.13745/j.esf.sf.2021.10.40

Abstract

Abstract:

Magmatic epidote can be used as a pressure gauge to reflect the emplacement depth of the rock mass, from which the rate of crustal uplift and denudation and the history of crustal evolution can be assessed. Magmatic epidote in Quangangintou gneissic tonalite from the Changle-Nan’ao structural belt is the fifth epidote occurrence reported in China and the first discovered in the southeast coastal area. The gneissic tonalite, formed in the Early Cretaceous (K₁), is dominated by calcic, medium-potassium, calc-alkaline series and metaluminous, weak peraluminous, calc-alkaline series, possessing the properties of magnesium andesite and showing the trondhjemite evolutionary trend. Light rare earth elements are relatively enriched in the gnessic tonalite, with no obvious negative Eu anomaly. Large Ta, P and Ti negative anomalies show the characteristics of volcanic arc granites in orogenic belts. As revealed by petrographic analysis, the biotite and automorphic epidote are distributed in the gaps in plagioclase forming a typical filling structure. The two minerals, wrap around each other, appear as primary minerals co-crystallized from the late water-rich magma, not secondary ones formed after the magmatic period of metasomatic plagioclase. By EPMA analysis, the power factor Ps for the epidote is between 24-29, and TiO₂ content is less than 0.1%. Petrographic and chemical analyses further reveal the epidote in gneissic tonalite is magmatic epidote, formed underground at a pressure of ~8.7 kbar and a depth of 25-32 km. Combining the above results with the crustal thickness of ~30 km for the present southeast coast continent, the corresponding Early Cretaceous (K₁) crustal thickness is estimated at about 55-62 km. After the Early Cretaceous (K₁), the crust might have experienced multiple uplifts.

Key words: magmatic epidote, discovery, geological significance, gneissic tonalite, Changle-Nan’ao structural belt

CLC Number:

CUI Xianyue, CHEN Bolin, SI Xiaobo, DENG Jinfu, FENG Yanfang, XIAO Qinghui. Magmatic epidote in Quanganglintou gneissic dacodiorite from the Changle-Nan’ao structural zone: Discovery and geological significance[J]. Earth Science Frontiers, 2022, 29(2): 234-240.

Figures/Tables 8

Fig.1 Geological map of the Putian-Quanzhou study area showing the distribution of sampling points. Adapted from 26.

Fig.2 Field characteristics of magmatic epidote-bearing gneissic tonalite. Adapted from 12.

Table 1 Major element compositions (100% without water) and CIPW norms in magmatic epidote-bearing gneissic tonalite

样品编号	w_B/%
样品编号	SiO₂		TiO₂	Al₂O₃		Fe₂O₃		FeO		MnO		MgO		CaO	Na₂O		K₂O		P₂O₅		Total
F11-3	69.31		0.48	14.92		1.97		2.04		0.09		1.39		4.26	4.09		1.27		0.17		100
F11-4	56.81		0.91	17.07		3.16		5.38		0.15		4.15		7.32	3		1.78		0.29		100
F11-5	59.96		0.84	16.47		2.66		4.72		0.17		3.75		5.66	3.39		2.11		0.29		100
样品编号	C.I.P.W.Norm矿物组分含量/%
样品编号	石英	钙长石			钠长石		正长石		刚玉		透辉石		紫苏辉石			钛铁矿		磁铁矿		磷灰石		合计
F11-3	29.55	18.6			34.59		7.53		0		1.17		4.41			0.91		2.86		0.39		100
F11-4	10.16	27.85			25.35		10.55		0		5.38		13.75			1.73		4.58		0.66		100.01
F11-5	13.43	23.52			28.65		12.44		0		2.18		13.65			1.6		3.86		0.66		99.98

Fig.3 Microscopic characteristics of gneissic tonalite (F11-3)

Fig.4 Microscopic characteristics of gneissic tonalite (F11-4)

Fig.5 Microscopic characteristics of gneissic tonalite (F11-5)

Table 2 Results of electron probe analysis of epidote in Quanganglintou gneissic tonalite

样品号	w_B/%
样品号	SiO₂		TiO₂			Al₂O₃			FeO			MnO			CaO			Na₂O			TOTAL
F11-3-3107	38.31		0.04			22.93			12.66			0.39			23.09			0.14			97.56
F11-4-1112	38.04		0			23.14			11.46			0.14			23.86			0			96.64
F11-5-1114	38.61		0			23.46			10.76			0.42			23.16			0			96.41
F11-5-3116	37.5		0.06			22.41			11.84			0.37			22.05			0			94.23
样品号	以12.5个O为基础计算的阳离子数																					Ps
样品号	Si	ZAl		ΣZ	Ti		Al	CR		Fe³⁺	Fe²⁺		Mg²⁺	Mn²⁺		ΣM	Ca²⁺		Na⁺	ΣA		Ps
F11-3-3107	3.026	0		3.026	0.002		2.134	0		0.831	0.005		0	0.026		2.999	1.954		0.021	1.975		27.8
F11-4-1112	3.026	0		3.026	0		2.169	0		0.779	0		0	0.009		2.958	2.033		0	2.033		26.4
F11-5-1114	3.076	0		3.076	0		2.203	0		0.646	0.071		0	0.028		2.948	1.977		0	1.977		22.5
F11-5-3116	3.068	0		3.068	0.004		2.161	0		0.697	0.113		0	0.026		3	1.933		0	1.933		24.2

Table 3 Representative microprobe data. Adapted from [2].

Pluton	样品号	w_B/%
Pluton	样品号	SiO₂	TiO₂	Al₂O₃	Fe₂ $O 3 *$	MgO	MnO	CaO	Na₂O	F	SUM	-F≡O	SUM
Round Valley,Idaho	Riggins #3	37.29	0.16	23.62	13.95	0	0.22	24.26	0.01	0	99.51	0	99.51
Ecstall,B.C.	JS-E2	37.51	0.05	25.99	12.4	0.14	0.34	23.01	0	0.03	99.47	0.01	99.46
Moth Bay,Alaska	AK-1	36.64	0.08	23.62	13.69	0	0.49	23.34	0	0.03	97.89	0.01	97.88

Table 3 Representative microprobe data. Adapted from [2].

Pluton	样品号	w_B/%
Pluton	样品号	SiO₂	TiO₂	Al₂O₃	Fe₂ $O 3 *$	MgO	MnO	CaO	Na₂O	F	SUM	-F≡O	SUM
Round Valley,Idaho	Riggins #3	37.29	0.16	23.62	13.95	0	0.22	24.26	0.01	0	99.51	0	99.51
Ecstall,B.C.	JS-E2	37.51	0.05	25.99	12.4	0.14	0.34	23.01	0	0.03	99.47	0.01	99.46
Moth Bay,Alaska	AK-1	36.64	0.08	23.62	13.69	0	0.49	23.34	0	0.03	97.89	0.01	97.88

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